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Protostars are stars that are about to be born. These are glowing clouds of interstellar gas and dust, which look like dark spots in the midst of light. Gravity causes every atom and every bit of dust to pull on every other one and all move to the center, causing the protostar to collapse. Having begun with a diameter of perhaps 1.5 trillion km, the protostar now shrinks at a very fast rate about 1,000 years, to a diameter of about 80 million km. Because the atoms move faster and faster as they fall toward the center, friction is created as they rub together and the temperature rises. The protostar starts at a temperature of about 100 K and over 1.000 years, rises to about 4,250 K. This heat causes the protostar to glow in with its own light, giving off even more light than our Sun even though it is not nearly as hot. After about 10,000 years, the protostar's surface temperature is up to about 4,500 K and it is now 100 times as luminous as the Sun; after another 100,000 years the temperature is 5,000 K even though the protostar has been shrinking the entire time and only gives off about 10 times the light of the Sun. There is little change in temperature over the next 10 million years but the brightness continues to drop as the protostar contracts. The next 20 million years is the last stage of the protostar's development where it equals the Sun in luminosity and its size becomes fixed at about 1.6 million km in diameter. 30 million years after the pocket of gas began to form, a star is born.


A star has is born when a protostar stops shrinking in size. At this moment the core temperature reaches 10 million K and a reaction, called nuclear fusion begins in the core. Nuclear fusion is the joining, or fusing, of small atomic nuclei to create larger ones. The nuclei of two hydrogen atoms join to form the nucleus of one helium atom and there is a great release of energy. We see and feel this energy in the form of light and heat from the Sun; all other stars give off the same energy but they are too far away for us to observe it. The fusion in a star continues for about 90 percent of its life, but cannot go on forever. Our Sun is expected to continue this way for just over another 5 billion years. The life cycle of a star can be compared to a human life: the 30 million years as a protostar is like infancy or early childhood, from birth to about age 4. The 10 billion year period of nuclear fusion as a bright star is the time of youth and maturity, from age 4 to 76. Last is old age. But while humans tend to grow smaller, the star grows larger. It also becomes red in colour and for these reasons is called a red giant.


Without the outward flow of energy from the star's core, gravity now takes over, squeezing and compressing the core making it grow much smaller. The core radiates heat until it reaches about 4 million K; it is now releasing great amounts of energy which carries immense amounts of hydrogen gas out to the surface. The star begins to grow larger, however, it does not become brighter. As a result of a temperature drop, the star now glows red instead of blue-white. Astronomers have located a number of red giants in the universe. Alderbaran is in the constellation Taurus, Antares in Scorpio, Arcturus in Bootes, and Betelgeuse in Orion. Most of the red giants are found in globular clusters, which are groups of up to one million stars that move together through space. Astronomers believe that the main source of carbon and oxygen in the universe is the very hot cores of red giants and these elements make like as we know it possible.


As the star cools off, a cycle begins. As a result of the temperature drop, the outer gas layer spreads out; this causes the temperature to drop further, and this in turn causes the gas to spread farther. It eventually spreads to far that it actually separates from the body of the star altogether. A cloud of glowing atoms moves out in all directions; this is called a planetary nebula.

Planetary Nebula: "Hubble 5"

It is called such because early astronomers believed it looked like a distant planet. Today we know that a planetary nebula has nothing to do with a planet, but still the name remains. The gas particles eventually mix with the clouds of gas normally found between the stars and the planetary nebula disappears. All that now remains of the star is the central core which, because it is no longer giving off energy, begins to collapse bit by bit. This process can be compared to squishing a large closetfull of clothes into a small suitcase. The clothes are still the same, they just take up much less room. Likewise, all the matter that was in the large star is still there, only now is is packed much more tightly. This occurs until the star is about the same size as planet Earth. Astronomers guess that a teaspoon of this matter weighs about 907 metric tonnes! The star is still very hot, so hot that it gives off a white light and thus is called a white dwarf. With the passage of billions of years the white dwarf cools off, from white hot to yellow, and then red, eventually becoming completely cold and black. Astronomers have located about 500 hundred white dwarfs and of the 20 stars closest to Earth, 2 are white dwarfs.

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